Michael Sam -- I Proposed ... At The Vatican!

Michael Sam clearly wanted to make a statement when he proposed to his longtime BF -- 'cause the football player popped the question right in Vatican City.

Sam posted the pic ... which shows HE was the one who got down on one knee and popped the question to Vito Cammisano last week.

We're told Sam and Vito were inside St. Peter's Basilica at the time.

It's sort of an act of defiance ... considering the Catholic church is still anti-gay marriage.

Mazel!